This invention relates generally to a system and method for placing implantable medical devices within a body; and more particular, relates to the use of audio signals for assessing status associated with an implantable medical device such as a lead during an implantation procedure.
Implantable medical devices such as electrical leads have long been employed in the fields of cardiac stimulation and monitoring. For example, leads are generally employed to deliver electrical stimulation for cardiac pacing and cardioversion/defibrillation applications. In these applications, endocardial leads are placed through a transvenous route to locate one or more sensing and/or stimulation electrodes in a desired location within a heart chamber or interconnecting vasculature. To provide effective therapy, electrodes carried at the lead distal end need to be accurately positioned at a predetermined location against the endocardium or within the myocardium. The lead distal tip is then generally affixed by a passive or active means to the tissue to maintain the desired location.
It is often difficult to determine whether a lead has been properly positioned and adequate tissue contact has been achieved. In some instances, catheters and leads are utilized that include materials that will allow for visualization with fluoroscopy. Additionally, fluoro-visible dyes may be injected into the cardiac chambers and venous anatomy so that the chambers of the heart and the related vasculature are visible using a fluoroscopic device. This procedure, sometimes referred to as a xe2x80x9cvenogramxe2x80x9d, allows the surgeon to locate a precise site and achieve proper electrode placement when performing an implant procedure.
Although the use of fluoro visible media is viable in some instances, this process has several disadvantages. First, some patients have adverse physical reactions when exposed to the fluoro visible dye used to obtain a venogram. Moreover, obtaining the venogram exposes the patient and clinicians to radiation. Additionally, a fluoroscope of the type needed for obtaining the fluoro-visible image may not be available. Finally, obtaining the venogram adds additional steps to the implant procedure, lengthening the time required to complete the procedure and increasing the risk of infection and complications to the patient.
An alternative method for determining the adequacy of tissue contact involves the use of impedance measurements. Since the impedance of fluids generally differs from that of tissue, impedance measurements may be used to determine whether an electrode has come in adequate contact with tissue.
Tissue impedance measurements are generally acquired by applying a known current or voltage signal to the tissue and measuring a resulting voltage or current, respectively. A system employing this method is disclosed in U.S. Pat. No. 5,447,529. A similar system is disclosed in U.S. Pat. No. 5,935,079 to Swanson et al., which describes a system to measure the electrical contact between the myocardium and multiple electrodes on a multiple electrode array. This Swanson system determines the amount of contact by transferring an electrical signal such as a current into the tissue so that tissue impedance may be measured.
Generally, when employing impedance measurements to assess tissue contact, the impedance signals are represented on some type of visual display for use by the clinician. However, because the impedance measurements and associated impedance changes are very small, the changes are difficult to detect visually. Moreover, because the physician is required to continually consult a monitor device for status, the physician must look away from the patient, diverting attention from the immediate task of manipulating the IMD into position. What is needed, therefore, is an improved system and method for assessing the status of an IMD such as a lead, and in particular, for assessing lead-to-tissue contact, during an implant procedure.
An improved system for monitoring the status of an IMD within a body is provided. In one embodiment, the IMD is a lead. The system includes a signal generator for supplying a first signal to the lead, which may be a constant current or voltage signal. A resulting voltage or current signal is generated, respectively, and may be sensed as an indication of the impedance of a portion of the body that is proximal to one or more electrodes carried by the lead. The impedance indication may then be converted to an audible signal. The audible signal has a frequency that is proportional to the amplitude of the impedance indication. The pitch of the audible signal therefore rises when measured impedance increases, and drops as the impedance decreases.
The inventive system provides an improved mechanism for monitoring the status of a lead or other IMD implanted within a body. Because impedance changes are converted to audible signals, the clinician need not utilize a visual display to detect the changes. This is advantageous because the clinician is allowed to focus all attention on manipulation of the IMD within the body, without having to divert attention to a display screen or other monitor. Additionally, the small changes in impedance are more readily detected using audible, rather than visual, means.
Many types of status indications may be detected using the current inventive system. For example, since the impedance between two electrodes at a lead distal tip changes as contact is made between the distal tip and cardiac tissue, tonal changes in the generated signal may be used to assess tissue contact. Moreover, impedance also changes as an active fixation device is embedded within tissue, causing tonal changes in the monitored audio signal useful for assessing the degree, and depth, of fixation. Similarly, the tonal changes may be used to detect over-torquing of a lead, which may cause wrapping of cardiac tissue around the lead tip such that impedance is increased. It may be further noted that tonal changes may be used to determine lead position within a body, since different types of tissue having different impedance values. Whether an extendable fixation mechanism is in a retracted or extended position may also be determined using tonal changes in the monitored audible signal. Finally, movement of a lead within a delivery catheter lumen can may be monitored using tonal changes in the monitored audio signal. For example, the tone changes as a tip and/or ring electrode carried by a lead are advanced beyond a catheter distal tip.
According to one embodiment of the invention, a system is provided for use in monitoring an implantable medical device (IMD) within a body. The system includes a first circuit to measure impedance of a portion of the body proximate a predetermined portion of the IMD. This first circuit may include a filter and amplifier to process the measured impedance indication. This signal may also be offset by a user-selectable amount during a calibration process, allowing the system to provide a predetermined audible signal when the IMD is in a predetermined state. This allows changes in the IMD status to be more readily detected.
According to another aspect of the invention, the system may include a processing circuit to process digitized samples of the measured impedance indication. Based on comparisons between the signal samples and predetermined criteria, the processing circuit develops an indication of the state of the IMD such as a lead and any associated fixation device. This indication may include information pertaining to tissue contact, the type of tissue being contacted, the extent of any tissue fixation, the position of an extendable/retractable fixation mechanism, and the position of an IMD relative to a delivery device such as a catheter. The data generated by the processing circuit may be provided to the user on an output device such as an LED display, a printer, or a display monitor. The data may be used to generate a virtual representation of the IMD within a patient""s body.
Another embodiment of the invention involves a method for monitoring an IMD located within a body, including the steps of measuring a signal indicative of impedance within a portion of the body proximal the IMD, and generating an audible signal representative of the measured signal. The audible signal is then used to determine status associated with the IMD in the manner discussed above.
Other scopes and aspects of the invention will become apparent from the drawings and the accompanying description.